Children screamed and parents ducked as the explosive detonated. Police said two officers had shrapnel wounds, one was hospitalized with hearing damage and a fourth suffered injuries that weren't immediately determined.

Three Protestant militants were arrested. The Red Hand Defenders, regarded by police as a cover name used by the Ulster Defense Assn., the outlawed Protestant group, claimed responsibility for the attack.

The grenade was thrown as about 100 young girls and their relatives walked through a Protestant neighborhood to the front door of Holy Cross Primary School under heavy police protection.

"It was awful that a policeman was hit, but it could have been one of those little girls. That has been my fear all along," said the Rev. Aidan Troy, governor of the school. He has urged Catholic parents in vain to use a rear entrance not threatened by the mob.

More than 40 police officers have been wounded since rioting began Monday in Ardoyne, a predominantly Catholic neighborhood. The besieged school lies in Ardoyne's small Protestant area, where residents insist no Catholics will be welcome until attacks on their own homes end.

Just before nightfall, Protestant youths on Ardoyne Road began throwing rocks, golf balls and debris over police lines. The opening salvos sent dozens of police and British soldiers scurrying for cover behind a row of armored vehicles that separate the two communities.

Tensions in Ardoyne have spread to several other polarized parts of north Belfast, the most bitterly divided side of Northern Ireland's capital.

Britain faced mounting pressure to take punitive action against the Ulster Defense Assn., which has been blamed for most of the recent attacks against Catholics.

The UDA whose flags fly from lampposts throughout the Protestant section of Ardoyne is supposed to be observing a cease-fire in support of the province's 1998 peace pact, and more than 200 members were paroled from prison as part of the deal.